1. Field of the Invention
The present invention relates to a superconducting radiation detector having a high energy resolution and a high counting rate by using the abruptness of a superconducting transition edge for converting a slight heat generated by an X-ray into a high signal current, and using an electrothermal self-feedback mechanism.
2. Description of the Related Art
Microcalorimeters using a superconducting transition edge have a higher energy resolution and a higher counting rate (hereinafter referred to as “microcalorimeter”) than detectors using semiconductors, such analyzers have been attracting attention for analysis of elements or impurity tests. Energy Dispersive Spectroscopy (EDS) is known, and use detectors according to the related are having semiconductors to quickly carry out analysis of elements for a wide range of energy. However, since energy resolution depends on the energy gap of a semiconductor, it cannot be below 100 eV. Thus, a microcalorimeter for use as a detector having both an improved energy resolution and a high counting rate is desirable. In a microcalorimeter, the temperature is set to the vicinity of the superconducting transition, a constant voltage is driven, and thus an electrothermal self-feedback mechanism functions to achieve a high energy resolution and a high counting rate. Details of microcalorimeters are described in K. D. Irwin, Applied Physics Letters 66, 1988 (1995). A superconducting transition temperature represents the temperature of transition from a normal conducting state to a superconducting state of a material. The vicinity effect by forming a thin film of a normal conductor on a superconductor shifts the superconducting transition temperature to a lower temperature as compared to that of a single layer. The film thickness ratio between the superconductor and the normal conductor determines the amount of shift in the transition temperature. In the case where the microcalorimeter has a bilayer structure comprised of a superconductor and a normal conductor, hot electrons generated by absorption of radiation mostly diffuse in the normal conductor. Diffusion of electrons should be as quick as possible, because a shorter diffusion length increases the time of temperature rise of the calorimeter Accordingly, the variation in peak values of waveforms of signal pulses is effected. Particularly, the phenomenon that the thinner the film is, the shorter the diffusion length of the electron is called the effect of thin film.
In a calorimeter, an absorber is provided on a resistor, and thus the area of the resistor is required to be larger than that of the absorber. To improve the efficiency of absorbing X-rays, it is required that the absorber is thick or the area thereof is large. Therefore, the area of the resistor cannot be smaller than that of the absorber.
In case that the absorber is a normal conductor, a part of the resistor under the absorber turns into the normal conducting state due to the vicinity effect, and the superconducting transition temperature is determined by a part of the resistor on the side opposite to the absorber. In case that the size of the resistor is fixed, the transition temperature of a calorimeter having an absorber is lower than that of a calorimeter having no absorber, which also causes a problem of a large width of transition. A large transition width degrades the energy resolution and the counting rate of the calorimeter.